it depends on the lawn mower, I have found them to generally be be between 17mm and 21mm. for the blades off the disc, and between 12mm and 14mm for the blades to the disc
I would probably use a half inch breaker bar on the blade jammed with a 2x4. And a 3/8 socket set to remove the motor. Some lawn mowers are metric while others are inch. Whatever fits snugly.